Satellites that orbit the earth in the equatorial plane at an altitude of 6.611 earth radii are geosynchronous, or relatively fixed above a particular point on earth. Since all geosynchronous satellites are in the equatorial plane (celestial latitude=0.degree.), the location of a particular satellite is unambiguously designated by its longitude. For example, geosynchronous satellites located on an arc between approximately 70.degree. West longitude and 135.degree. West longitude provide coverage for the continental United States.
Earth-mounted dish antennas can receive data from any geosynchronous satellite within its longitudinal scope. It is relatively straightforward to direct a dish antenna to a particular geosynchronous satellite by rotating the receiving dish about its north-south polar axis. In addition, the parabolic reflector axis is then pointed toward the Clark Belt along the geosynchronous arc by adjusting the declination angle. Such systems operate on the theory that rotation about an axis which is normal to the plane passing through the site and two geosynchronous satellites will aim the antenna at the two satellites with minimal pointing error. At the equator, the polar axis is parallel to the surface of the earth, and the dish antenna, mounted to rotate about that axis, faces straight up. At non-equatorial installations, the angle of the polar axis is equal in magnitude to the latitude of the installation site. In the Northern Hemisphere, a dish antenna mounted to rotate about its polar axis faces South toward the celestial equator, while a dish in the Southern Hemisphere faces North toward the celestial equator. In each case, the polar axis of the dish is normal to the equatorial plane.
In a single axis system of the type described, a dish antenna can contact any geosynchronous satellite it can point to. At present, geosynchronous satellites positioned on the arc described above ring much of the earth, with approximately 4.degree. of separation between each. As such, it is a goal of dish antenna designers to maximize the number of satellites a dish can track, by encompassing as much of the geosynchronous orbit arc as possible, up to the theoretical horizon-to-horizon sweep limit of 180.degree.. While such dish antennas can sweep across a 180.degree. arc, they can only point to satellites on a 162.6.degree. arc, at the equator, and to smaller arcs at higher latitudes. These limits arise because of encounters with the horizon at either end of the arc.
At present, either of two fundamentally different dish antenna drive assembly types are used to orient a dish to desired longitudes. To date, only one of the two types can sweep out a horizon-to-horizon arc of 180.degree.. Existing horizon-to-horizon assemblies rely upon either fine-tooth gearing or a system of chains and sprockets to move the dish across the 180.degree. arc. While horizon-to-horizon type drives sweep out the largest possible arc, they are typically much more complex and expensive than linear actuator type drives.
Typical linear actuator drives have a moveable shaft pivotally connected at one end to the underside of the dish. The movable shaft is typically a screw jack operated manually or by motor drive that urges the mounted dish antenna around its polar axis. At most, the linear actuator type of drive assembly can sweep out an arc of about 110.degree., because of the mechanical constraints that arise from connecting the linear actuator directly to the dish and to the mount.
What is lacking, therefore, in the art is a dish antenna drive assembly that combines the simplicity and economy of existing linear actuator drives with the wide-range survey capability of existing horizon-to-horizon drives.